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A Quantum Cascade Laser (QCL) is a semiconductor laser operating in the mid to long infrared and terahertz regime. In QCL, the electron is responsible for the emission of photon tunnels in the following quantum well and, consequently, several photons can be generated by a single electron, which makes them extremely efficient. The transition from one quantum well to another is the origin of the term "quantum cascade".
Their wide tuning range and fast response time allow for faster and more precise trace element detectors and gas analyzers that are replacing slower FTIR, mass spectroscopy, and photothermal microspectroscopy.
The Quantum Cascade Laser was first demonstrated in 1994. Remarkable effort has been put into making them more robust, versatile, and manufacturable. The number of markets for QCL is growing at an ever-increasing rate as researchers and manufacturers gain more knowledge with them.
Standoff Photoacoustic Detection
The basic mechanism of photoacoustic sensing involves directing a laser at the chemical to be detected. On absorption, wave sounds are produced and collected by the microphone equipment from a safe distance. Figure 1 represents the schematic of photoacoustic detection. The recovered signal from the microphone is then processed for evaluation of chemical composition. Thus, the photoacoustic detection could safely detect explosive devices, leaks of harmful gases, airborne chemicals and, medical diagnostics as well as preventing terrorist activities.
The various benefits of QCL-based absorption spectrometers include high sensitivity, high selectivity, non-destructive, compactness, portability and real-time monitoring. Therefore, QCL systems have widely used in the measurement of trace tropospheric chemical species complex gas mixture.
The QCL based absorption spectrometer provides a unique molecular fingerprint for monitoring air pollutants in urban areas. A wide variety of gaseous species were measured from stationary and mobile platforms.
Breath Analysis for Medical Diagnostic
The QCL based breath analysis can be applied to health monitoring and diagnosis of asthma, cystic fibrosis, and chronic obstructive pulmonary disease. The presence of high levels of methane, acetone, ethane, nitric oxide, carbon monoxide and carbonyl sulfide in the breath, can be used as a biological marker of these diseases.
For instance, methane and acetone are markers of the intestinal environment condition and diabetes, while the presence of nitric oxide and carbon monoxide provides clues as airway inflammation. In addition, the presence of carbonyl sulfide provides a possible biological marker of cystic fibrosis. Analysis of exhaled breath has the potential for non-invasive identification and monitoring of person’s wellness states.
Directed Infrared Countermeasures (DIRCM)
QCL is in a unique position to provide new mid-infrared spectroscopy output to disorient an incoming threat missile, hence breaking the lock with the intended target and driving it off course. DIRCM systems are designed to protect aircraft from heat-seeking missiles. In most typical DIRCM architectures, optical parametric oscillator shifted solid state or fiber lasers provide the output used to defeat these threats.
In many cases, aircraft use flames to confuse heat-seeking missiles and bait them from their intended targets, but newer and smarter generations of air defense systems are able to differentiate between flames and aircraft; therefore, rendering flames much less effective.
The terahertz (THz) laser sources have potential applications in radio astronomy, high-sensitivity gas sensing and spectroscopy, security camera screening, and biomedical imaging, non-invasive techniques of industrial inspection of processes. The variety of security and public safety applications mainly arise from the fact that many materials, such as clothing material, plastics, biological materials, and packaging materials, are semi-transparent or transparent at THz frequency range.